Wireless underground sensor networks (WUSNs), which enable real-time sensing and monitoring of underground resources by underground devices (UDs), hold great promise for delivering substantial social and economic benefits across various verticals. However, due to the harsh subterranean environment, scarce network resources, and restricted communication coverage, WUSNs face significant challenges in supporting sustainable massive machine-type communications (mMTC), particularly in remote, disaster-stricken, and hard-to-reach areas. To complement this, we conceptualize in this study a novel space-air-ground-underground integrated network (SAGUIN) architecture that seamlessly incorporates satellite systems, aerial platforms, terrestrial networks, and underground communications. On this basis, we integrate LoRaWAN and wireless energy transfer (WET) technologies into SAGUIN to enable sustainable subterranean mMTC. We begin by reviewing the relevant technical background and presenting the architecture and implementation challenges of SAGUIN. Then, we employ simulations to model a remote underground pipeline monitoring scenario to evaluate the feasibility and performance of SAGUIN based on LoRaWAN and WET technologies, focusing on the effects of parameters such as underground conditions, time allocation, LoRaWAN spread factor (SF) configurations, reporting periods, and harvested energy levels. Our results evidence that the proposed SAGUIN system, when combined with the derived time allocation strategy and an appropriate SF, can effectively extend the operational lifetime of UDs, thereby facilitating sustainable subterranean mMTC. Finally, we pinpoint key challenges and future research directions for SAGUIN.

翻译：无线地下传感器网络（WUSNs）通过地下设备（UDs）实现对地下资源的实时感知与监测，有望为多个垂直领域带来显著的社会与经济效益。然而，由于地下环境恶劣、网络资源稀缺以及通信覆盖受限，WUSNs在支持可持续的大规模机器通信（mMTC）方面面临巨大挑战，尤其是在偏远、受灾及难以抵达的区域。为此，本研究提出一种新型的空天地井一体化网络（SAGUIN）架构，该架构无缝融合了卫星系统、空中平台、地面网络与地下通信。在此基础上，我们将LoRaWAN与无线能量传输（WET）技术整合至SAGUIN中，以实现可持续的地下mMTC。首先，我们回顾了相关技术背景，阐述了SAGUIN的架构与实施挑战。随后，通过仿真模拟远程地下管道监测场景，评估基于LoRaWAN与WET技术的SAGUIN的可行性与性能，重点关注地下条件、时间分配、LoRaWAN扩频因子（SF）配置、上报周期以及能量收集水平等参数的影响。结果表明，所提出的SAGUIN系统结合推导出的时间分配策略与合适的SF，能够有效延长UDs的工作寿命，从而促进可持续的地下mMTC。最后，我们指出了SAGUIN面临的关键挑战与未来研究方向。